Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of integrated circuits. Sputtering was originally used to deposit generally planar layers of material on a wafer and was particularly used for depositing aluminum electrical interconnect lines. However, in recent years, the emphasis and challenges have shifted to depositing materials used in vertical interconnections in high aspect-ratio vias and similar vertically oriented structures formed in and through dielectric layers. Copper metallization has further changed the emphasis since bulk copper can be easily deposited by electrochemical plating (ECP). However, various thin liner layers are required prior to the ECP, for example, barrier layers, such as Ta and TaN, to prevent the copper from migrating into the oxide dielectric and copper seed layers to provide a plate electrode and to initiate the growth of the copper ECP layer.
Techniques have been developed to allow the sputter deposition of thin uniform layers onto the walls of high aspect-ratio holes. One such technique that has met significant commercial success is self-ionizing plasma (SIP) sputtering in which a significant fraction of the sputtered atoms are ionized and thus can be electrostatically attracted deep within narrow holes. It is called self-ionizing plasma because some of the sputtered ions are attracted back to the sputtering target to sputter yet more atoms or ions, thereby reducing the need for an argon working gas and allowing sputtering at a lower pressure. The extreme of SIP is sustained self-sputtering (SSS) in which the sputtered ions are sufficient to maintain the sputtering plasma and hence the argon can be removed.
A conventional PVD chamber 10, with a few modifications for SSS or SIP sputtering, is illustrated schematically in cross section in FIG. 1. The illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The chamber 10 includes a vacuum chamber body 12 sealed through a ceramic insulator 14 to a sputtering target 16 having at least a front face composed of the material, usually a metal, to be sputter deposited on a wafer 18 held on a heater pedestal electrode 20 by a wafer clamp 22. Alternatively to the wafer clamp 22, a cover ring or an electrostatic chuck may be incorporated into the pedestal 20 or the wafer may be placed on the pedestal 20 without being held in place. The target material may be aluminum, copper, aluminum, titanium, tantalum, cobalt, nickel, molybdenum, alloys of these metals containing less than 10 wt % of an alloying element, or other metals and metal alloys amenable to DC sputtering. On the other hand, RF sputtering may be used to sputter material from a dielectric target. A grounded shield 24 held within the chamber body 12 protects the chamber wall 12 from the sputtered material and provides a grounded anode. A selectable and controllable DC power supply 26 negatively biases the target 14 to about −600VDC with respect to the shield 24. Conventionally, the pedestal 20 and hence the wafer 18 are left electrically floating, but for most types of SIP sputtering, an RF power supply 28 is coupled to the pedestal 18 through an AC capacitive coupling circuit 30 or more complex matching and isolation circuitry to allow the pedestal electrode 20 to develop a DC self-bias voltage in the presence of a plasma. A negative DC self-bias attracts positively charged sputter ions created in a high-density plasma deeply into a high aspect-ratio holes characteristic of advanced integrated circuits. Even when the pedestal 20 is left electrically floating, it develops some DC self-bias.
A first gas source 34 supplies a sputtering working gas, typically argon, to the chamber body 12 through a mass flow controller 36. In reactive metallic nitride sputtering, for example, of titanium nitride or tantalum nitride, nitrogen is supplied from another gas source 38 through its own mass flow controller 40. Oxygen can alternatively be supplied to produce oxides such as Al2O3. The gases can be admitted from various positions within the chamber body 12. For example, one or more inlet pipes located near the bottom of the chamber body 12 supply gas at the back of the shield 24. The gas penetrates through an aperture at the bottom of the shield 24 or through a gap 42 formed between the cover ring 22 and the shield 24 and the pedestal 20. A vacuum pumping system 44 connected to the chamber body 12 through a wide pumping port 46 maintains the interior of the chamber body 12 at a low pressure. Although the base pressure can be held to about 10−7 Torr or even lower, the conventional pressure of the argon working gas is typically maintained at between about 1 and 100 milliTorr. However, for self-ionized sputtering, the pressure may be somewhat lower, for example, down to 0.1 mTorr. For sustained self-sputtering, particularly of copper, once the plasma has been ignited, the supply of argon may be stopped, and the chamber pressure may be made very low. A computer-based controller 48 controls the reactor including the DC power supply 26 and the mass flow controllers 36, 40.
When the argon is admitted into the chamber, the DC voltage between the target 16 and the shield 24 ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively biased target 16. The ions strike the target 16 with a substantial energy and cause target particles to be sputtered from the target 16. Some of the target particles strike the wafer 18 and are thereby deposited on it, thereby forming a film of the target material. In reactive sputtering of a metallic nitride, nitrogen is additionally admitted into the chamber body 12, and it reacts with the sputtered metallic atoms to form a metallic nitride on the wafer 18.
To provide efficient sputtering, a magnetron 50 is positioned in back of the target 16. It includes opposed magnets 52, 54 coupled by a magnetic yoke 56 to produce a magnetic field within the chamber in the neighborhood of the magnets 52, 54. Typically in SIP sputtering, the magnetron 50 is small, nested, and unbalanced with one or more inner magnets 52 surrounded by opposed outer magnets 54 of greater magnetic intensity. The magnetic field traps electrons and, for charge neutrality, the ion density also increases to form a high-density plasma region 58 within the chamber adjacent to the magnetron 50. To achieve uniform sputtering onto the wafer 18, the magnetron 50 is usually rotated about the center 60 of the target 16 by a shaft 62 driven by a motor 64. Typical rotation speeds are 50 to 100 rpm. In a conventional magnetron, the shaft 62 is fixed with respect to the magnets 52, 54 and is coincident with the target center 60 so that the magnetron 50 sweeps a constant track about the target center 60.
Fu in U.S. Pat. No. 6,306,265 discloses several designs of a magnetron useful for SSS and SIP. For these applications, the magnetron should produce a strong magnetic field and have a small area. Rotating the magnetron can nonetheless provide uniform sputter deposition and full target erosion if desired. The magnetron should include an inner pole associated with one or more inner magnets 52 surrounded by a continuous outer pole of the opposite polarity associated with the outer magnets 54. The inner and outer poles are unbalanced in the sense that the total magnetic flux produced by the outer pole is substantially greater than that produced by the inner pole by a factor of at least 1.5. Thereby, magnetic field lines from the outer pole 54 extend deeply into the chamber towards the wafer 16. The power supplied by the DC supply 26 to the target 16 should be high, of the order of 20 kW for a 200 mm wafer. However, scaling the power supply for 300 mm wafers presents some difficulties. Nonetheless, the combination of high power and small magnetron area produces a very high power density beneath the magnetron 50 and hence a moderately high-density plasma area 58 without the use of supplemental plasma source power, such as would be provided by RF inductive coils. The form and size of the magnetron 50 are related to some aspects of the invention.
To counteract the large amount of power delivered to the target, the back of the target 16 may be sealed to a backside coolant chamber 66. Chilled deionized water 68 or other cooling liquid is circulated through the interior of the coolant chamber 66 to cool the target 16. The magnetron 50 is typically immersed in the cooling water 68, and the target rotation shaft 62 passes through the back chamber 66 through a rotary seal 70.
Such an SIP chamber 10 can be used for both sputtering the barrier layer, for example, of TaN/Ta from a tantalum target and sputtering the thin copper seed layer from a copper target. Particularly for barrier layers, continuous and symmetrical deposition with the structure is critical for minimum sidewall coverage requirement and via bottom thin-down/punch-through process. Barrier sputtering has been found to be optimized for relatively small magnetrons that concentrate on the peripheral region or edge of the target with little or no effective sputtering in the target center. Material eroded from the target periphery region possesses preferred incident angles to achieve symmetrical step coverage. In addition, the small magnetron produces a high power density and hence high ionization fraction with a relatively low DC power supply. However, target erosion at the periphery of the target will produce re-deposition around the central area of the target, which central area is not on net eroded. The re-deposition needs to be eliminated during the sputtering or cleaning process. The cleaning process will be described below. Uniform target erosion and high average sputtering rate in the case of Cu and Al deposition are not primary considerations for the small amount of material being sputtered for the thin barrier or seed on each wafer.
For SIP sputtering with magnetrons not extending over an entire radius of the target 16, the rotating magnetron 50 does not scan the entire area of the target 16 and sputtered material tends to redeposit on the non-scanned areas. Copper redeposition does occur but is not generally considered to be a significant problem since the redeposited copper bonds relatively well with the copper target. Barrier redeposition, however, may present a significant problem. Part of the barrier sputtering may occur in a nitrogen ambient in a process called reactive sputtering to deposit a metal nitride layer, such as TaN or TiN, on the wafer. The nitride also redeposits on the metal target and grows in thickness over many wafer cycles. Such redeposited material is prone to flaking and hence presents a source of particles. As a result, it is often considered necessary to prevent the redeposited barrier material from flaking, preferably by preventing its growth beyond a critical thickness.
Rosenstein et al. (hereafter Rosenstein) in U.S. Pat. No. 6,228,236 has presented a solution for the redeposited material. They affix their magnetron 50 to an eccentric arm such that centrifugal force can be controlled to cause the magnetron to assume two radial positions depending upon the rotational direction of the magnetron drive shaft 62. Rosenstein effectively interposes a radial translation mechanism 74 between the rotation drive shaft 62 and the magnetron 50. He is primarily concerned with redeposition on the target periphery outside the operational area of the target. His design provides a small radial translation of the magnetron and the orientation of their magnetron to the rotation circumference remains substantially unchanged between the two positions. Also, the switching depends at least in part on hydrodynamics. The Rosenstein design could be modified to enable cleaning the target center, but this would rely on a reversal of the magnetron rotation. It is instead desired to provide a magnetron design which avoids the need to reverse the direction of rotating the magnetron, thereby quickening the transition between multiple radial rotation diameters. Such a design would desirably minimize the time necessary for achieving a clean mode position so as to maximize the fraction of time for wafer deposition, in order to obtain high throughput.
Various types of planetary magnetrons have been proposed, see for example U.S. Pat. application Ser. No. 10/152,494, filed May 21, 2002 by Hong et al. and now issued as U.S. Pat. No. 6,852,202, and U.S. patent application Ser. No. 10/418,710, filed Apr. 17, 2003 by Miller et al. and now issued as U.S. Pat. No. 6,841,050, both of which are commonly assigned with the present application. The disclosed planetary mechanisms are capable of scanning a small magnetron over substantially all of the target surface in a planetary path produced by two rotational arms. Although planetary scanning in a single though convolute path can be used to avoid redeposition, it is nonetheless often desired to confine the primary sputtering to a relatively narrow radial range of the target.